Note: You can read Greek intelligently without mastering the material presented in this section. If you plan to travel back to classical Athens in a time machine, some of this information may help you make yourself understood in the marketplace. Then again, it may not. We know much less than we would like to admit about the spoken language, particularly during the Golden Age of Athens. The system explained below is primarily based on the practice of scribes in the 9th century CE — a point in time as far removed from Plato as it is from the present day. The epigraphic evidence (variant spellings for words painted on pottery or carved into marble) offers some insight into the pronunciation of individual letters but sheds no light on the actual music of the language. Be that as it may... we will pretend that there is certainty, and plunge on.
The Basic Rule of Accentuation:
Word accent depends on the length of the last syllable of a word.
If the last syllable is LONG, then the accent goes on the syllable before it.
If the last syllable is SHORT, then the accent goes two syllables before it.
Words that do not follow this rule always have written accent marks in these online editions of the Greek classics. Words that do follow the rule have no written accents in these editions, except in those cases where an accent mark helps to disambiguate homographic forms.
(See the section on Poetic Meter for an explanation of syllable length, but be aware that -aι and -oι at the end of a word count as short for the accent rule, even though they count as long for poetic meter.)
The Music of Language
Languages sound different not only because of variations in the consonants and vowels, but also because each language tends to make use of certain characteristic musical phrases defined by pitch. Those musical phrases differ from language to language, creating an identifiable "accent" associated with its speakers. (A foreigner singing a song has little or no accent, because the melody displaces the natural music of their native tongue.)
The defining pitches of ancient Greek were:
1) A base pitch or "tonic"; and
2) An emphatic high pitch 3½ tones above the tonic note (the interval called a "fifth").
The music of the language of ancient Greece resided in the interplay between those two tones, with the rises in tone tending to be abrupt while the falls in tone could be either abrupt or gradual.
The fundamental musical phrase in the spoken language of ancient Greece was this:
Tonic + Fifth + Tonic + short Tonic
Another important phrasing was:
Fifth + Tonic + Tonic + Fifth.
The Alexandrian System of Written Accentuation
Alexandrian scholars in the 2nd century BCE devised a system of diacritical markings to indicate the pitch accents of Greek words. Those accent marks originally served to differentiate ambiguous words that could have different meanings depending on which syllable received the accent. They were also employed to indicate word accents that would be unpredictable to a speaker of Latin.
The three accent marks are:
1) The acute accent, ά έ ί ό ύ, an abrupt rise in pitch from the tonic to the musical fifth above it (3½ tones up).
2) The grave accent, ὰ ὲ ὶ ὸ ὺ, an abrupt lowering in pitch from the fifth to the tonic below it (3½ tones down).
3) The circumflex accent, ᾶ ῆ ῖ ῶ ῦ, a rise in pitch to the fifth followed by a slow glide back down to the tonic note.
Most words take the accent according to the following rules:
1) If the last syllable is short, then the accent falls two syllables before it (provided the word has that many syllables).
2) If the last syllable of a word is long, then the accent falls on the syllable just before it.
NOTE: Some words are accented on the final syllable, rather than trying to push the accent forward to the beginning of the word.
Since such words have no following syllable to allow the pitch to return to the tonic, they take the grave accent instead of the acute whenever they are followed by another word.
The accented syllable starts at the same high pitch as the acute, but then swoops down to the tonic, allowing the following word to start on the tonic as required.
What Dionysius of Halicarnassus Tells Us About Ancient Greek
We owe an enormous debt to an ancient professor of public speaking, Dionysius of Halicarnassus, a Greek who was teaching in Rome near the end of the 1st century BCE. He not only preserved one of Sappho's poems for us (Fragment 1) but he gave us our most important clues about the pronunciation of classical Greek. We also owe a debt to the anonymous 11th-century scribe who wrote out Περι συνθεσεως ονοματων on calfskin, preserving it for us.
Here is what Dionysius of Halicarnassus has to say about the pitch accent of classical Greek:
Note: I highly recommend Stephen Usher's edition of the Critical Essays of Dionysius of Halicarnassus in the Loeb Classical Library. There are many volumes in the bilingual Loeb Library whose scholarship is impeccable and whose translations are precise, and Dr. Usher's is among them.